Techniques to improve current regulator capability to protect the secured circuit from power side channel attack

ABSTRACT

This disclosure relates to current flattening circuits for an electrical load. The current flattening circuits incorporate randomize various parameters to add noise onto the supply current. This added noise may act to reduce the signal to noise ratio in the supply current, increasing the difficulty of identifying a computational artifact signal from power rail noise.

BACKGROUND

In computer security, a side-channel attack is any attack based oninformation gained from the implementation of a computer system, ratherthan weaknesses in the implemented algorithm itself (e.g., cryptanalysisand software bugs). Timing information, power consumption, andelectromagnetic leaks can provide information on the operation of thesystem that may be exploited.

Some side-channel attacks utilize technical knowledge of the internaloperation of the system, while others, such as differential poweranalysis, are effective as black-box attacks. Differential poweranalysis (DPA) is a side-channel attack that involves analyzing powerconsumption measurements from a system, often using statistical methods.The attack exploits leakage of internal power consumption informationback to supply pins external to the circuit package. Varying powerconsumption can occur when microprocessors or other secure hardwareperform secure operations such as cryptographic algorithms using secretkeys. Using DPA, an adversary can obtain information at the package pinsregarding power consumption from multiple operations performed by asecure load in an electronic device.

One approach to protecting against side-channel attacks utilizescircuits that prevent the power fluctuations that arise fromcomputations in protected circuits from propagating back to pins of thecircuit package, where such fluctuations may be exploited. Circuitsproviding such prevention are known as “current flattening circuits”.

BRIEF SUMMARY

This disclosure relates to a current flattening circuit for anelectrical load. A constant current source is coupled between the loadand a power supply. A randomizer perturbs the constant current sourceoutput. A controlled current shunt is configured to respond to a loadvoltage to generate a shunt current for the load.

This disclosure further relates to a current flattening circuitcomprising a current shunt and a comparator each coupled in parallelwith a load. A reference voltage is coupled to one or more inputs of thecomparator, and a randomizer is configured to perturb the referencevoltage. The comparator is configured to control the current shunt.

This disclosure also relates to a current flattening circuit with atleast one transistor interposed between a power supply and a load. Acurrent shunt is coupled to a load voltage node between the transistorand the load. A comparator is coupled to the load voltage node and to areference voltage and is coupled to control the current shunt. Arandomizer is coupled to perturb a clock driving the load and in someimplementations, also to the reference voltage.

Additionally, this disclosure relates to a circuit comprising a load, adummy load, and a switch. The load is configured to perform atransformation of a true input. The dummy load is configured to performa transformation of a dummy input. The switch is configured to direct aclock edge to either the load or the dummy load.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

To easily identify the discussion of any particular element or act, themost significant digit or digits in a reference number refer to thefigure number in which that element is first introduced.

FIG. 1 depicts a current flattening circuit 100 in accordance with oneembodiment.

FIG. 2 depicts a current flattening circuit 200 in accordance with oneembodiment.

FIG. 3 depicts a current flattening circuit 300 in accordance with oneembodiment.

FIG. 4 depicts a current flattening circuit 400 in accordance with oneembodiment.

FIG. 5 depicts a randomized clocking circuit 500 in accordance with oneembodiment.

FIG. 6 depicts a dummy loaded circuit 600 in accordance with oneembodiment.

DETAILED DESCRIPTION

Embodiments of circuits are disclosed incorporating a current regulatorinto current flattening circuits to detect the transient behavior of aload current, process the detected load current, and to generate aregulated current that is complimentary current. “Complimentary current”refers to a current mirroring amplitude variations of another current.The sum of the load and regulated currents gives the supply loadcurrent, which in an ideal case may be direct current. The alternatingcurrent value of this supply load current may be treated as being zeroat all frequencies in an ideal circuit designed to protect againstside-channel attacks.

The disclosed circuits further incorporate a randomizer. “Randomizer”refers to a circuit that generates a randomized signal. Herein, thisalso refers to circuits that generate pseudo-random signals. Randomizingvarious parameters within the current flattening circuits may add noiseonto the supply current. This added noise may act to reduce the signalto noise ratio in the supply current, increasing the difficultly of adata-revealing signal being detected from power rail noise. By varyingdifferent parameters within the current flattening circuits, signal tonoise ratios in the power supply current may be reduced, addingadditional security. The disclosed circuits incorporate noise additionby variation of reference in a current regulator, variation of loadclock frequency to add timing noise, and combination of both (i.e.,reference variation and load clock variation).

FIG. 1 depicts current flattening circuit 100 in one embodiment. At ahigh level, the current flattening circuit 100 comprises a constantcurrent source 102, a high bandwidth current regulator 104, and a filtercapacitor 106 configured to protect a circuit load 108 from side-channelattacks. A parasitic inductance due to package architecture is modeledas package inductance 110 between the power supply 112 and the constantcurrent source 102. “Constant current source” refers to a circuitconfigured to regulate its output current toward a constant level thatis determined by its input control signal(s). “Current regulator” refersto a circuit that generates a control input signal to a constant orcontrolled current source.

The high bandwidth current regulator 104 comprises a differentialamplifier 114 that compares the load voltage V_(load) with a referencevoltage Vref and generates a proportional control output to the gate ofa controlled current shunt 116, such as Shunt transistor 118. Thecontrolled current shunt 116 generates a shunt current 120, i.e., acomplimentary current of load current AC components, and thus preventsthese components from propagating back to the power supply 112 packagepin. “Controlled current shunt” refers to a circuit configured to outputan amount of shunt current based on its input control signal(s).

The constant current source 102, generating a load current 122, may beimplemented using a stack of transistors in series, with finite outputimpedance r₀. The high bandwidth current regulator 104 mitigates theleakage of load voltage/load current information through r₀ by shuntingload current transients to ground. The filter capacitor 106 helpsmaintain V_(load) at a constant level in the presence of the dynamicshunt current 120 (complimentary current) that is generated by the highbandwidth current regulator 104.

FIG. 2 depicts a current flattening circuit 200 in an embodiment inwhich the constant current source 102 comprises a PMOS transistor 202 orPMOS transistor stack 204. At least one PMOS transistor 202 may beinterposed between the power supply 206 and the circuit load 208. ThePMOS transistor 202 may be electrically modelled, conceptually, as aconstant current source in parallel with a resistance r₀. This outputimpedance r₀ of the constant current source 102 may be improved bystacking one or more pass-gate transistors.

The bias voltage V_(bias) for the constant current source 102 may begenerated by a bandgap reference generator 230. A digital-to-analogconverter 214 may have inputs coupled to V_(bias) and a random code RNDgenerated by a random signal generator. The random code may be generatedat a rate that matches the bandwidth of the current regulator 216. Theoutput of the digital-to-analog converter 214 may control the constantcurrent source 102, for example by driving the gate or gates of the PMOStransistor 202 or PMOS transistor stack 204, respectively. Thedigital-to-analog converter 214 thus configured may act as a randomizerto perturb the constant current source 102.

A parasitic inductance due to package architecture is modeled as packageinductance 210 between the power supply 206 and the constant currentsource 102. A capacitor 212 may be configured between the power supply206 and ground to provide a high frequency noise path to ground fornoise on the power supply 206 line. The capacitor 212 may also modelpackage parasitic capacitance.

A current regulator 216 may comprise an NMOS transistor 218 and adifferential amplifier 220 each coupled to a node 222 between the atleast one PMOS transistor 202 and the circuit load 208. The NMOStransistor 218 may act as a shunt transistor. The shunt transistor gatevoltage may be configured to be controlled by the differential amplifier220. The differential amplifier 220 may be coupled to compare areference voltage 224 and a load voltage 226. A filter capacitor 228 maybe configured between the node 222 and ground in order to provide a pathto ground for noise on the load voltage 226 line.

FIG. 3 depicts a current flattening circuit 300 in one embodiment. Thecurrent flattening circuit 300 comprises elements in common with thecurrent flattening circuit 200 but also differs in the followingrespects.

The current flattening circuit 300 may incorporate a digital-to-analogconverter 302 coupled to the reference voltage 224 and a randomizedsignal RND as part of the current regulator 216. The digital-to-analogconverter 302 output may be coupled to one input of the differentialamplifier 220, the load voltage 226 being coupled to the other input ofthe differential amplifier 220. The output of the differential amplifier220 may control the gate of NMOS transistor 218, which may act as acontrolled current shunt, similar to the configuration illustrated inFIG. 2 .

The current regulator 216 acts as a controlled current source configuredto respond to the load voltage 226. The NMOS transistor 218 isconfigured to receive a gate voltage proportional to a comparison of thereference voltage 224 and the load voltage 226. The reference voltage224 may be a bandgap voltage input to a digital-to-analog converter 302,e.g., from the bandgap reference generator 230 used to generateV_(bias), and a randomizer may be configured by randomly perturbing theoutput of the digital-to-analog converter 302 to the differentialamplifier 220.

Depending on the implementation, the current flattening circuit 300 mayutilize randomization at the PMOS transistor 202, at the differentialamplifier 220, or both.

FIG. 4 depicts a current flattening circuit 400 in one embodiment. Thecomprises elements in common with the current flattening circuit 200 butalso differs in the following respects. In the current flatteningcircuit 400 illustrated, the constant current source 102 may becontrolled by the bias voltage V_(bias) directly, e.g., as the gatevoltage of one or more PMOS transistor 202, rather than V_(bias) beingfirst transformed by a digital-to-analog converter (such asdigital-to-analog converter 214 shown in FIG. 2 ). In this configurationa randomized signal RND may be coupled to perturb the circuit load 208.RND may be a randomized clocking signal generated by the randomizedclocking circuit 500 illustrated in FIG. 5 . The use of a randomizedclocking signal in this fashion may be used as depicted in the currentflattening circuit 400 or complementary with the embodiments depicted inFIG. 2 or FIG. 3 .

A comparator for controlling the shunt current is implemented in thecurrent regulator 216 using a differential amplifier 220 with inputterminals coupled to a reference voltage 224 and the load voltage 226(the reference voltage 224 source may be generated by a bandgap voltagesource). Alternatively, the comparator may be implemented as depicted inFIG. 4 .

FIG. 5 depicts a randomized clocking circuit 500 in one embodiment. At ahigh level, the randomized clocking circuit 500 comprises a phase lockedloop 502, an edge blocking circuit 504, a comparator 506, and a counter508. In the depicted embodiment the high-level components are coupled asfollows. A reference clock signal “Refclk” is coupled to the input ofthe phase locked loop 502. The output of the phase locked loop 502 is aclock signal “clkin” coupled to one input of the edge blocking circuit504. One input of a comparator 506 is coupled to the counter 508 andanother input of the comparator 506 receives an N-bit random number. Thecomparator 506 compares these inputs and asserts an output to the edgeblocking circuit 504 when the counter 508 value does not meet acomparison metric (e.g., is not equal to the N-bit random number), atwhich point the counter 508 value may be reset. The edge blockingcircuit 504 perturbs the clock driving the circuit load 208 by blockinga clocking edge of clkin when the comparator 506 output is asserted toproduce a randomized clock output “Rand_Clk”. The Rand_Clk signal drivesthe circuit load 208 as depicted in FIG. 4 .

The phase locked loop 502, may be configured to generate “clkin” at afrequency higher than a clock frequency at which the circuit load 208operates. The edge blocking circuit 504 generates a randomized blockingsignal responsive to the output of the comparator 506, based on counter508 and the N-bit random number.

Those of ordinary skill in the art may readily recognize that therandomized clocking circuit 500 may comprise additional or differentelements not depicted and unnecessary to this description. Therandomized clocking circuit 500 may be incorporated into any of theembodiments depicted in FIG. 2 -FIG. 4 .

FIG. 6 depicts a dummy loaded circuit 600 in one embodiment. At a highlevel, the dummy loaded circuit 600 comprises a constant current source102, a controlled current shunt 116, a load 602, a dummy load 604, and arandomizer 608.

The controlled current shunt 116 responds to a load voltage to generatea shunt current or a proportional shunt current for one or both of theload 602 and dummy load 604. Whether or not the controlled current shunt116 is coupled to both of the load 602 or the dummy load 604, or both,may be selected according to the parameters of the particularimplementation. The load 602 transforms a true input (depicted as “plaintext”, the actual sensitive data to process) into the cipher text(“text” is meant in the cryptography sense, not literally to meantextual input, although it could be). The dummy load 604 performs atransformation of a dummy input (depicted as “random text”) that draws acomparable amount of current. In some embodiments the dummy load 604 isa replica of the load 602. The dummy input may also be randomized. Thedummy load 604 depicted transforms a random plain text input. Theoperations performed by the load 602 and/or the dummy load 604 need notbe cryptographic in nature; in general, they may perform any type ofsignal processing. Regardless of the nature of the secure computationsto obfuscate, the dummy load 604 should perform processing that isdifficult to distinguish from the processing performed by the load 602.

The clock is directed to either the load 602 or the dummy load 604depending on the randomized output state of the switch control generator606.

The randomizer 608 in the depicted embodiment comprises a switch controlgenerator 606, three inverters, and two two-input NAND gates. Therandomizer 608 accepts a clock signal as input (CLK) to the switchcontrol generator 606 and to a pair of NAND gates. The switch controlgenerator 606 randomly generates a ‘1’ or a ‘0’ output. The output ofthe switch control generator 606 drives the other input terminals of theNAND gates. CLK 1 to the load 602 and CLK 2 to the dummy load 604 aregenerated from the outputs of the NAND gates.

Various functional operations described herein may be implemented inlogic that is referred to using a noun or noun phrase reflecting saidoperation or function. For example, an association operation may becarried out by an “associator” or “correlator”. Likewise, switching maybe carried out by a “switch”, selection by a “selector”, and so on.

Within this disclosure, different entities (which may variously bereferred to as “units,” “circuits,” other components, etc.) may bedescribed or claimed as “configured” to perform one or more tasks oroperations. This formulation—[entity] configured to [perform one or moretasks]—is used herein to refer to structure (i.e., something physical,such as an electronic circuit). More specifically, this formulation isused to indicate that this structure is arranged to perform the one ormore tasks during operation. A structure can be said to be “configuredto” perform some task even if the structure is not currently beingoperated. A “credit distribution circuit configured to distributecredits to a plurality of processor cores” is intended to cover, forexample, an integrated circuit that has circuitry that performs thisfunction during operation, even if the integrated circuit in question isnot currently being used (e.g., a power supply is not connected to it).Thus, an entity described or recited as “configured to” perform sometask refers to something physical, such as a device, circuit, memorystoring program instructions executable to implement the task, etc. Thisphrase is not used herein to refer to something intangible.

The term “configured to” is not intended to mean “configurable to.” Anunprogrammed FPGA, for example, would not be considered to be“configured to” perform some specific function, although it may be“configurable to” perform that function after programming.

Reciting in the appended claims that a structure is “configured to”perform one or more tasks is expressly intended not to invoke 35 U.S.C.§ 112(f) for that claim element. Accordingly, claims in this applicationthat do not otherwise include the “means for” [performing a function]construct should not be interpreted under 35 U.S.C § 112(f).

As used herein, the term “based on” is used to describe one or morefactors that affect a determination. This term does not foreclose thepossibility that additional factors may affect the determination. Thatis, a determination may be solely based on specified factors or based onthe specified factors as well as other, unspecified factors. Considerthe phrase “determine A based on B.” This phrase specifies that B is afactor that is used to determine A or that affects the determination ofA. This phrase does not foreclose that the determination of A may alsobe based on some other factor, such as C. This phrase is also intendedto cover an embodiment in which A is determined based solely on B. Asused herein, the phrase “based on” is synonymous with the phrase “basedat least in part on.”

As used herein, the phrase “in response to” describes one or morefactors that trigger an effect. This phrase does not foreclose thepossibility that additional factors may affect or otherwise trigger theeffect. That is, an effect may be solely in response to those factors,or may be in response to the specified factors as well as other,unspecified factors. Consider the phrase “perform A in response to B.”This phrase specifies that B is a factor that triggers the performanceof A. This phrase does not foreclose that performing A may also be inresponse to some other factor, such as C. This phrase is also intendedto cover an embodiment in which A is performed solely in response to B.

As used herein, the terms “first,” “second,” etc. are used as labels fornouns that they precede, and do not imply any type of ordering (e.g.,spatial, temporal, logical, etc.), unless stated otherwise. For example,in a register file having eight registers, the terms “first register”and “second register” can be used to refer to any two of the eightregisters, and not, for example, just logical registers 0 and 1.

When used in the claims, the term “or” is used as an inclusive or andnot as an exclusive or. For example, the phrase “at least one of x, y,or z” means any one of x, y, and z, as well as any combination thereof.

Having thus described illustrative embodiments in detail, it will beapparent that modifications and variations are possible withoutdeparting from the scope of the invention as claimed. The scope ofinventive subject matter is not limited to the depicted embodiments butis rather set forth in the following Claims.

What is claimed is:
 1. A circuit comprising: a load configured toperform a transformation of a true input; a dummy load configured toperform a transformation of a dummy input; and a switch configured todirect a clock edge to either the load or the dummy load.
 2. The circuitof claim 1 wherein the dummy input is randomized.
 3. The circuit ofclaim 1, further comprising: a controlled current source configured torespond to a load voltage to generate a shunt current for the load. 4.The circuit of claim 1, further comprising: a controlled current sourceconfigured to respond to a load voltage to generate a shunt current forthe load; and a randomizer coupled to perturb the controlled currentsource.
 5. The circuit of claim 1, the switch configured to alternatebetween directing the clock edge to the load and to the dummy load. 6.The circuit of claim 1, wherein the switch is configured to direct theclock edge to either the load or the dummy load according to arandomized signal.
 7. The circuit of claim 1, further comprising: acontrolled current source configured to respond to a load voltage togenerate a shunt current for the dummy load.
 8. The circuit of claim 1,further comprising: a controlled current source configured to respond toa load voltage to generate a shunt current for both of the load and thedummy load.
 9. A system comprising: a first circuit configured toperform a first cryptographic transformation of plain text into ciphertext; a second circuit configured to perform a second transformation ofa randomized input into cipher text; and a third circuit configured torandomly alternate clock edges to either the first circuit or the secondcircuit.
 10. The system of claim 9 wherein one or both of the firstcircuit and the second circuit receive an output of a controlled currentshunt.
 11. The system of claim 10, the controlled current shuntconfigured to respond to a load voltage of the first circuit to generatea shunt current for the first circuit.
 12. The system of claim 10,wherein a shunt current generated by the controlled current shunt israndomized.
 13. The system of claim 9, the third circuit comprising:logic to transform a clock signal into a randomized series of logical‘1’ and logical ‘0’ values.
 14. The system of claim 9, furthercomprising: the controlled current shunt configured to respond to a loadvoltage of the first circuit to generate a shunt current for the secondcircuit.
 15. The system of claim 9, further comprising: the controlledcurrent shunt configured to respond to a load voltage of the firstcircuit to generate a shunt current for the first circuit and for thesecond circuit.
 16. A method comprising: performing a first computationwith a first circuit; performing a second computation with a secondcircuit; and alternately clocking either the first circuit or the secondcircuit in a random fashion.
 17. The method of claim 16, whereinalternately clocking either the first circuit or the second circuit in arandom fashion comprises directing a clock edge from a clock circuit toeither the first circuit or the second circuit during each clock cycle.18. The method of claim 16, further comprising: randomizing the secondcomputation.
 19. The method of claim 16, further comprising: varying ashunt current to the first circuit, the second circuit, or both.
 20. Themethod of claim 19, the shunt current varied according to a voltagegenerated to perform the first computation.
 21. The method of claim 16,further comprising: supplying both of the first circuit and the secondcircuit from a same constant current source.